Earthquake tremors (and damage caused by such tremors) are the result of three basic types of elastic wave caused by the slipping of plates in the earth's crust against each other; two of these waves are capable of traveling through rock. The first of these three waves is the primary or P wave; this wave is a compression wave and propagates linearly in the direction of travel through rock and fluid; this is the fastest traveling seismic wave. The secondary or S wave generally moves more slowly than the P wave and its wave movement is at right angles (up and down, and/or side-to-side) to the direction of travel. It is the S wave that causes most damage to structures.
The third type of wave is called a surface wave, and is restricted to the ground surface. This type of wave has a motion similar to ripples on the surface of water. There are two types of surface waves. The first is called a Love wave and is similar to that of an S wave having a side-to-side motion with little or no vertical displacement; these waves can cause substantial damage to objects since virtually all the energy is employed within a horizontal plane. The second type of surface wave is called a Rayliegh wave, which is like an ocean wave and can cause displacement in both the vertical and horizontal plane relative to the direction of travel.
P and S waves have a characteristic which further affects shaking: when these waves move through layers of rock in the crust they are reflected or refracted at the interfaces between rock types. Whenever either wave is refracted or reflected, some of the energy of one type is converted to waves of the other type. For an example, as a P wave travels upwards and strikes the bottom of a layer of alluvium, part of its energy will pass upward through the alluvium as a P wave and part will pass upward as the converted S-wave motion. This means that the direction of shaking (e.g., left to right, front to back, or diagonally) in a given location is usually not entirely predictable, as it is dependent upon factors including the direction of wave travel and the nature (such as the density and homogeneity) of the crust in the general location in which the shaking is to be experienced. This in turn depends upon the location of the fault whose rupture has caused the waves.
Two approaches have been traditionally utilized to prevent or limit damage or injury to objects or payloads due to seismic events. In the first approach, used particularly with structures themselves, the objects or payloads are made strong enough to withstand the largest anticipated earthquake. However, in addition to the relative unpredictability of damage caused by tremors of high magnitude and long duration and of the directionality of shaking, use of this method alone can be quite expensive and is not necessarily suitable for payloads to be housed within a structure.
In the second approach the objects are isolated from the vibration such that the objects do not experience a major portion of the seismic waves. In certain cases, isolation flooring, for example “earthquake isolation flooring”, has been used or proposed. Such flooring has generally comprised a combination of some or all of the following features: a sliding plate, a support frame slidably mounted on the plate with low friction elements interposed therebetween, a plurality of springs and/or axial guides disposed horizontally between the support frame and the plate, a floor mounted on the support frame through vertically disposed springs, a number of dampers disposed vertically between the support frame and the floor, and a latch to secure the vertical springs during normal use.
Certain disadvantages to such pre-existing systems include the fact that it is difficult to establish the minimum acceleration at which the latch means is released; it is difficult to reset the latch means after the floor has been released; it may be difficult to restore the floor after it has once moved in the horizontal direction; the dissipative or damping force must be recalibrated to each load; there is a danger of rocking on the vertical springs; and since the transverse rigidity of the vertical springs cannot be ignored with regard to the horizontal springs, the establishment of the horizontal springs and an estimate of their effectiveness, are made difficult.
Ishida et al., U.S. Pat. No. 4,371,143 have proposed a sliding-type isolation floor that comprises length adjustment means for presetting the minimum acceleration required to initiate the isolation effects of the flooring in part by adjusting the length of the springs.
Yamada et al., U.S. Pat. No. 4,917,211 discloses a sliding type seismic isolator comprising a friction device having an upper friction plate and a lower friction plate, the friction plates having a characteristic of Coulomb friction, and horizontally placed springs which reduce a relative displacement and a residual displacement to under a desired value. The upper friction plate comprises a material impregnated with oil, while a lower friction plate comprises a hard chromium or nickel plate.
Stahl (U.S. Pat. No. 4,801,122) discloses a seismic isolator for protecting e.g., art objects, instruments, cases or projecting housing comprising a base plate connected to a floor and a frame. A moving pivoted lever is connected to a spring in the frame and to the base plate. The object is placed on top of the frame. Movement of the foundation and base plate relative to the frame and object causes compression of the lever and extension of the spring, which then exerts a restoring force through a cable anchored to the base plate; initial resistance to inertia is caused due to friction between the base plate and the frame.
Kondo et al., U.S. Pat. No. 4,662,133 describes a floor system for seismic isolation of objects placed thereupon comprising a floor disposed above a foundation, a plurality of support members for supporting the floor in a manner that permits the movement of the floor relative to the foundation in a horizontal direction, and a number of restoring devices comprising springs disposed between the foundation and the floor member. The restoring members comprise two pair of slidable members, each pair of slidable members being movable towards and away from each other wherein each pair of slidable members is disposed at right angles from each other in the horizontal plane.
Stiles et al., U.S. Pat. No. 6,324,795 disclose a seismic isolation system between a floor and a foundation comprising a plurality of ball and socket joints disposed between a floor and a plurality of foundation pads or piers. In this isolation device, the bearing comprises a movable joint attached to a hardened elastomeric material (or a spring); the elastic material is attached on an upper surface of the ball and socket joint and thus sandwiched between the floor and the ball and socket joint; the bearing thus tilts in relation to the floor in response to vertical movement. The floor is therefore able to adjust to buckling pressure due to distortion of the ground beneath the foundation piers. However, the device disclosed is not designed to move horizontally in an acceleration-resisting manner.
Fujimoto U.S. Pat. No. 5,816,559 discloses a seismic isolation device quite similar to that of Kondo, as well as various other devices including one in which a rolling ball is disposed on the tip of a strut projecting downward from the floor in a manner similar to that of a ball point pen.
Bakker, U.S. Pat. No. 2,014,643, is drawn to a balance block for buildings comprising opposed inner concave surfaces with a bearing ball positioned between the surfaces to support the weight of a building superstructure.
Kemeny, U.S. Pat. No. 5,599,106 discloses ball-in-cone bearings. Kemeny, U.S. Pat. No. 7,784,225 discloses seismic isolation platforms containing rolling ball isolation bearings. Hubbard et al., U.S. Patent Publication 2007/0261323, filed on Mar. 30, 2007 discloses a method and raised access flooring structure for isolation of a payload placed thereupon. Isolation bearings are disclosed in U.S. patent application Ser. No. 13/041,160 filed on Mar. 4, 2011, and Moreno et al., International Patent Application No. PCT/US11/27269, filed on Mar. 4, 2011.
All patents, patent applications and other publications cited in this patent application are hereby individually incorporated by reference in their entirety as part of this disclosure, regardless whether any specific citation is expressly indicated as incorporated by reference or not.